Growing demand for integrated circuits (ICs), especially microprocessors, with ever higher levels of performance and functionality have driven these devices to circuit densities beyond 100 million transistors per die. And this number may soon exceed one billion transistors on a single die. The growth in transistor density has been made possible by the use of MOSFET transistors with gate lengths below 100 nm. As gate length has shortened, power supply voltages have fallen, in some cases, to below 1 V.
High-speed microprocessors, with clock speeds above 3 GHz, may require in excess of 100 watts of power when operating at peak load. With operating voltages below 1 V, this translates to power supply currents that reach beyond 100 A. Additionally, the current requirements may change from idle (<20 A) to full power in a small number of clock cycles, leading to current transitions (di/dt) exceeding 30 GA/s.
In many applications, these high power integrated circuits operate at a very low duty cycle. For example, a microprocessor in a computer running a word processing application may only be required to operate for a few milliseconds each time a key on the user's keyboard is struck (less than once every 100 milliseconds). This is equivalent to a duty cycle of a few percent or less. In such applications, there is an opportunity to save significant power by slowing or stopping the microprocessor clock and reducing the supply voltage to minimize leakage current. Saving power is especially important in battery-powered devices such as PDAs, cellphones, laptop computers, and the like.
More recently, with the advent of so-called “multicore” processors (two or more processing units on a single die), there is an additional need (as well as opportunity) to reduce operating power during idle periods on one or more of the processing cores.
Integrated circuits are typically powered from one or more DC supply voltages provided by external supplies and high-efficiency, programmable DC-to-DC power converters located near the IC package. The power is provided through pins, leads, lands, or bumps on the integrated circuit package. Traditionally, to reduce the power to an integrated circuit, the power supply would be instructed, through its programming input, to reduce the supply voltage to a low level (often below 0.5 volts) or to zero volts. Because power converters have output filter and decoupling capacitance as high as several millifarads, power cycling much beyond 100 Hz will not yield significant power savings.